Conventional CAIPIRINHA is described in “Controlled aliasing in parallel imaging results in higher acceleration (CAIPIRINHA) for multi-slice imaging”, Magnetic Resonance in Medicine, Volume 53, Issue 3, pages 684-691, March 2005, by Felix A. Breuer, Martin Blaimer, Robin M. Heidemann, Matthias F. Mueller, Mark A. Griswold, & Peter M. Jakob. Conventional CAIPIRINHA is also described in U.S. Pat. No. 7,002,344, issued Feb. 21, 2006, which describes, in Col. 9, lines 27-31, how “if a phase-modulated RF excitation is used i.e., if the pulse phases are modulated from one two-slice excitation to the next as (0°,0°=++) and (0°,180°=+−), a shifting of both slice data sets effectively ensues in the phase coding direction.”
U.S. Pat. No. 7,002,344 FIG. 10c illustrates and Col. 12, lines 60-65 recite “a possible transfer to a segmented two-slice TrueFISP experiment. The first half of the acquisitions is alternatively provided with a pulse phase cycle (++,−−) while the other half is provided with a modulated pulse cycle (+−,−+). After the data acquisition, the acquired data are arranged according to their phase coding. Thus, a shifting of both slices against one another is achieved.” Thus, conventional CAIPIRINHA has generally been described with respect to modulating a pulse phase cycle. However, additional experimentation has yielded improvements and specific embodiments not contemplated in the general description.
Magnetic resonance imaging (MRI) may employ parallel imaging techniques. Some parallel imaging techniques may produce under-sampling aliasing artifacts. These artifacts may be removed using a post-processing image reconstruction algorithm. These artifacts may also be mitigated using CAIPIRINHA. CAIPIRINHA modifies the appearance of aliasing artifacts to improve subsequent parallel image reconstruction. CAIPIRINHA has been shown to be more efficient than some other multi-slice parallel imaging concepts that rely solely on a post-processing approach.
In CAIPIRINHA, multiple slices of arbitrary thickness and distance are excited simultaneously using multi-band radiofrequency (RF) pulses. Data is then under-sampled, which produces superimposed slices that appear shifted with respect to each other. The shift between aliased slices can be controlled by modulating the phase of the individual slices in the multi-band excitation pulse from echo to echo.
TrueFISP (True Fast Imaging with Steady State Precession) is a coherent imaging technique that uses a balanced gradient waveform. Because it uses balanced gradient waveforms, TrueFISP may be referred to as a balanced steady state free precession technique. TrueFISP image contrast is determined by T2/T1 properties and depends primarily on TR (repetition time). As gradient hardware has continued to improve, shorter and shorter TRs are becoming available, which makes TrueFISP of continuing interest. TrueFISP relies on balanced gradient moments per TR and a short TR to reduce banding artifacts that may appear in an acquired image.
Even though TrueFISP is an inherently fast imaging sequence, there is always a need to image faster. For example, cardiac imaging and real-time imaging can be improved with faster imaging. Also, patients that cannot perform breath-holds appreciate faster imaging. One way to speed up image acquisition is to use parallel imaging where multiple slices are acquired simultaneously. However, conventional TrueFISP has TRs that are so short that it may be difficult, if even possible at all, to acquire interleaved slices.
In theory, a TrueFISP sequence might be accelerated by including two dimensional (2D) parallel imaging that acquires multiple slices simultaneously. However, incorporating 2D multi-slice parallel imaging and TrueFISP is also limited by the steady state requirements of TrueFISP. Additionally, reducing the number of phase encoding lines and acquiring slices with very small separations may lead to undesirable signal-to-noise ratio (SNR) loss.